A conventional lamp assembly is shown in FIG. 1. The lamp assembly consists of a bulb 11, a holder 12, a socket seat 13, copper tabs 14 and a power cable 15. The bulb 11 is placed in the holder 12 with its leads 111 passing through and turned up and being against outer sides of the holder 12 (not shown in FIG. 1). The copper tabs 14 are arranged against inner sides of the socket seat 13. When the holder 12 of the bulb 11 is placed in the socket seat 13, the turned up leads 111 will contact with the copper tabs 14 respectively to form an electrical connection. However in using the lamp assembly, the leads 111 which are turned up and against the holder 12 will come off occasionally; furthermore, on engagement of the holder 12 and the socket seat 13 may be broken due to a gap between them, or the electric circuit disconnection will occur for the uncontacted copper tab 14 with leads 111 further, the bulb 11 has a shorter service life of ca 1000 hours, so, some improvement has been obtained by using a light emitting element (light emitting dipolar substance) as a substitute for the bulb 11 in order to prolong the service life up to 50,000 hours. Although substitution of a light emitting element for the bulb 11 may prolong the service life, the light emitting element must have polarity which matches the polarity of the copper tabs when assembling thus causing an inconvenient assembling process. For example, the lamp assembly 10 is usually used in a lamp string. If one of the lamp assemblies 10 has its polarity misconnected, the lamp string circuit will not work, and it will take a lot of time to find out the wrong point. Therefore, the shortcomings mentioned above will result in an inconvenient assembling process and application. The present invention is directed to an improved lamp assembly featuring easy assembling and excellent fitting.